Rhenium-bound tungsten carbide composites

ABSTRACT

Rhenium metal is used as the sole binder metal in preparing hard metals based upon tungsten carbide, titanium carbide or hafnium carbide. The resultant hard metals can be used in conventional applications such as cutting tools.

This invention generally concerns cemented refractory metal matrixcomposites. This invention particularly concerns rhenium-bound tungstencarbide, rhenium-bound hafnium carbide and rhenium-bound titaniumcarbide metal matrix composites. This invention more particularlyconcerns the use of substantially pure rhenium metal as a binder fortungsten carbide, hafnium carbide or titanium carbide.

Cemented carbides, also known as "hard metals", conventionally include abasic carbide, such as tungsten carbide, and an iron group metal, suchas cobalt, as a binder. U.S. Pat. No. 4,432,794 discloses binder metalalloys that comprise a solid alloy of a transition metal of Group IVb,Vb or VIb with Re, Ru, Rh, Pd, Os, Ir or Pt. Lisovsky et al., in"Structure of a Binding Phase in Re-Alloyed WC-Co Cemented Carbides",Refractory Metals & Hard Materials, Volume 10 (1991), pages 33-36,discuss the effects of alloying WC-Co cemented carbides with rhenium.

Tools made of cemented carbide, including WC-Co (tungstencarbide-cobalt) as a typical composition, are widely used in themachining field. The alloy compositions, characteristics, uses andapplications of such cemented carbide materials are summarized inCemented Carbides for Engineers and Tool Users, International CarbideData (1983 ).

SUMMARY OF THE INVENTION

The present invention is a dense refractory composition-consistingessentially of rhenium in an amount within a range of from 1 to 25percent by weight of composition and a refractory metal carbide selectedfrom tungsten carbide, hafnium carbide and titanium carbide in an amountwithin a range of from 75 to 99 percent by weight of composition.

The dense refractory compositions can be used in any one of a number ofconventional applications for hard metals. One such application is as acutting tool used in machining or cutting a variety of materials such asmetals, plastics and wood products. Cutting tools include indexableinserts, end mills, router bits, reamers, drills, saw blades and knives.

DESCRIPTION OF PREFERRED EMBODIMENTS

The refractory metal carbide is suitably tungsten carbide, hafniumcarbide or titanium carbide. Tungsten carbide yields particularlydesirable results. The tungsten carbide has an average grain size thatis suitably about ten micrometers or less, beneficially about fivemicrometers or less, desirably about one micrometer or less, andpreferably from 0.4 to 0.8 micrometer. Acceptable grain sizes for othercarbides approximate those for tungsten carbide and can be readilydetermined without undue experimentation. The refractory metal ispreferably in powder or particulate form.

Rhenium metal powder suitable for purposes of the present invention hasa particle size that is desirably about five micrometers or less andpreferably from about two to about three micrometers (μm).

Rhenium metal powder and a refractory metal carbide powder are suitablyconverted to a powdered admixture by any one of a number of conventionalmixing processes. The use of an attritor, wherein balls of a hardmaterial, such as tungsten carbide/cobalt, are used to facilitatemixing, provides particularly satisfactory results.

The powdered admixture consists essentially of rhenium metal powder anda refractory metal carbide powder. In other words, no additional bindermetal, such as cobalt, is needed. The rhenium metal powder is present inan amount within a range of from 1 to 25 percent by weight, based uponpowdered admixture weight. The refractory metal carbide powder ispresent in an amount within a range of from 75 to 99 percent by weight,based upon powdered admixture weight. Desirable amounts of rhenium metalpowder and refractory metal carbide powder are, respectively, from 5 to20 and from 95 to 80 percent by weight of composition. Preferred amountsof rhenium metal powder and refractory metal carbide powder are,respectively, from 6 to 18 and from 94 to 82 percent by weight ofcomposition. The amounts of rhenium metal powder and refractory metalcarbide powder total 100 percent.

Mixing of the powders in an attritor is beneficially accomplished withthe aid of a liquid or solvent such as heptane. In order to facilitategreenware formation subsequent to mixing, a binder such as paraffin waxcan be added during the final stages of attrition. The attrited mixtureis desirably dried before further processing. Particularly satisfactoryresults are obtained by screening or classifying the attrited and driedmixture to remove unwanted agglomerates and fines.

Selection of a procedure for conversion of the screened powder mixtureis not particularly critical and depends largely upon desired shape ofthe greenware. Uniaxial pressing in hard tooling, dry or wet bag coldisostatic pressing in rubber tooling, extrusion and injection moldingare all used to form greenware in the hardmetals industry. For purposesof the present invention, uniaxial pressing in hard tooling, and dry orwet bag cold isostatic pressing in rubber tooling produce satisfactoryresults.

Before a pressed greenware article can be consolidated, it must first beprocessed to remove binders. In addition, thermal processing can beused, with a reducing gas such as hydrogen, to remove some metal oxides.At temperatures greater than those used to remove metal oxides, thermalprocessing provides preliminary, but limited or incomplete,densification by sintering. The term "incomplete densification" meansthat the greenware article, following thermal processing, has a densitythat is less than about 10%, typically 1-3%, greater than that of thegreenware article prior to thermal processing. These processes arebeneficially accomplished by programmed heating in a furnace undervacuum up to a temperature of about 350° C. for dewaxing or binderremoval, then under a hydrogen atmosphere over a temperature range offrom 350° C. to 525° C. to remove metal oxides and then under a vacuumat a temperature of about 1400° C. for a period of about two hours topartially sinter, or accomplish incomplete densification of, thegreenware. The dewaxed parts are then desirably wrapped in graphite foilto facilitate part recovery following densification.

Densification or consolidation of the dewaxed greenware is suitablyaccomplished by a procedure known as Rapid Omnidirectional Compaction(hereinafter "ROC"). ROC, as described by C. A. Kelto et al., in "RapidOmnidirectional Compaction (ROC) of Powder", Annual Review of MaterialScience, Volume 19, pages 527-50 (1989), is a quasi-isostaticconsolidation process used to densify powders or cold-pressed preforms.The process employs a conventional forging press and closed-die toolingto apply pressure to a preheated assembly called a fluid die.

U.S. Pat. No. 4,744,943, at column 5, line 13 through column 6, line 15,discloses pressure transmitting media and combinations of pressure,temperature and time used in a typical ROC process. Pressuretransmitting media include glass and certain salts that are flowable atpressures and temperatures used for consolidation. Boron-containingglass and Vycor™ Number 7913 brand glass are preferred pressuretransmitting media. Temperatures range from 400° C. to 2900° C.Pressures range from 10,000 psi (68.9 megapascals (MPa)) to a pressureat which the material being consolidated fractures (also known as the"fracture point"). The pressures are desirably between 50,000 psi (345MPa) and the fracture point, preferably between 70,000 psi (482 MPa) andthe fracture point and most preferably between 100,000 psi (689 MPa) andthe fracture point. The maximum pressure is preferably less than about500,000 psi (3450 MPa). Time at pressure is sufficient for the materialbeing consolidated to reach at least 85 percent of its theoreticaldensity. The time ranges from 0.01 second to 1 hour and is beneficiallyless than 30 minutes, desirably less than about 10 minutes, preferablyless than about 1 minute and most preferably less than about 20 seconds.

Prior to forging, the fluid die assembly containing the greenware isheated in an inert atmosphere to a temperature that is sufficient toyield a desired density. Suitable temperatures for tungstencarbide/rhenium admixtures range from 1600° C. to 1900° C. Desirabletemperatures range from 1650° C. to 1800° C. Temperatures for admixtureswherein hafnium carbide or titanium carbide replace tungsten carbide arereadily determined without undue experimentation.

Following forging or densification, the fluid die assembly containingrefractory metal/rhenium parts are suitably air cooled to ambienttemperature. The parts are then recovered from the die assembly byconventional means.

The densified parts suitably have a density greater than about 98percent of theoretical density. "Theoretical density" is based upon astraight line rule of mixtures. The density is desirably greater thanabout 99 percent of theoretical density. The density is preferably about100 percent of theoretical density.

The following examples illustrate the present invention. They do not,either expressly or implicitly, limit the scope of the invention. Allparts and percentages are by weight unless otherwise stated.

EXAMPLE 1

Tungsten carbide (87.52%) and rhenium metal (12.48%) powders are mixedin an attritor for four hours using tungsten carbide-cobalt millingmedia and heptane as a solvent. Paraffin wax (2%) is added to act as agreenware binder. The resultant mixture is dried and screened through a20 mesh (Tyler equivalent designation) (850 μm sieve aperture) screen.Greenware parts are made by cold-pressing the mixture that passesthrough the screen in steel tooling at 5,000 psi (35 MPa). Thecold-pressed parts are cold isostatically pressed at 30,000 psi (210MPa). The resultant parts are thermally processed, as disclosed herein,to dewax, remove metal oxides from, and partially sinter the greenware.The dewaxed greenware is wrapped in graphite foil, placed into a glasspocket fluid die (isostatic die assembly), preheated at 10° C./minute to1800° C., held at that temperature for 15 minutes and then isostaticallypressed (subjected to rapid omnidirectional compaction (ROC)) at 120,000psi (830 MPa) for 20 seconds using a forging press. The fluid die iscooled in air and the parts are recovered.

The recovered parts have the following properties: Density--15.95 g/cm³(99.0% of theoretical based upon a linear rule of mixtures); Hardness(Rockwell A) 94.8; Hardness (Vickers, 70 pound (32 kg) load) 2412 kg/mm²; Palmqvist Toughness (W) 30.6 kg/mm; and Hot Hardness (1200° C.) 11.99GPa.

EXAMPLE 2

The procedure of Example 1 is duplicated, save for reducing the preheattemperature from 1800° C. to 1650° C., for a 380 gram quantity of thetungsten carbide/rhenium mixture. The recovered parts)have a density of16.02 g/cm³ (99.4% of theoretical) and a Wear Number (ASTM G-65), usingwear bars having a length of 1.5 inch (3.8 cm) rather than 3 inches (7.6cm), of 910^(cm-3).

EXAMPLES 3-6

The procedure of Example 1 is replicated for tungsten carbide/rheniummetal mixtures having different rhenium metal contents and either thesame or different preheat temperatures. The rhenium metal contents andpreheat temperatures are shown in the following Table together withdensity, Vickers Hardness, and Palmqvist Toughness values for thedensified parts. Data from Example 1 is included for comparison.

    __________________________________________________________________________              Preheat                                                                            Density                                                             Rhenium                                                                            Temper-                                                                            (%   Vickers                                                                             Palmqvist                                                                           Fracture                                      Example                                                                            Content                                                                            ature                                                                              Theor-                                                                             Hardness                                                                            Toughness                                                                           Toughness                                     Number                                                                             (%)  (°C.)                                                                       etical)                                                                            (kg/mm.sup.2)                                                                       (kg/mm)                                                                             (MPa · m.sup.1/2)                    __________________________________________________________________________    1    12.48                                                                              1800 99.0.                                                                              2412  30.6  7.3                                           3    6    1650 99.0 2453  20.8  6.1                                           4    6    1800 98.9 2380  20.1  5.9                                           5    18   1650 98.9 2306  21.5  6.0                                           6    18   1800 99.3 2357  19.2  5.8                                           __________________________________________________________________________

The data from the Table demonstrate that pure rhenium metal serves as aneffective binder for tungsten carbide, at least in the amounts shown inthe Examples. Similar results are expected with greater amounts ofrhenium up to 25 percent by weight of composition. Similar results arealso expected when hafnium carbide or titanium carbide is substitutedfor tungsten carbide.

What is claimed is:
 1. A dense refractory composition consisting ofrhenium in an amount within a range of from 1 to 25 percent by weight ofcomposition and a refractory metal carbide selected from tungstencarbide, hafnium carbide and titanium carbide in an amount within arange of from 75 to 99 percent by weight of composition, the amounts ofrhenium and refractory metal carbide totaling 100 percent.
 2. Thecomposition of claim 1 wherein the refractory metal carbide is tungstencarbide in an amount within a range of from 80 to 95 percent by weightof composition and the amount of rhenium is from 5 to 20 percent byweight of composition.
 3. The composition of claim 1 wherein therefractory metal carbide is tungsten carbide in an amount within a rangeof from 82 to 94 percent by weight of composition and the amount ofrhenium is from 6 to 18 percent by weight of composition.
 4. Thecomposition of claim 2 having a density of greater than 98 percent oftheoretical density, based upon a linear rule of mixtures, a hardness(Rockwell A) greater than 90 and a wear number (ASTM G-65) greater than800^(cm-3).